1. Field of the Invention
The present invention relates to a color display device using an electroluminescent device and a method for manufacturing the same.
2. Related Art
In recent years, thin and lightweight flat panel displays have been substituted for cathode ray tubes (CRTs) in various fields, and their applications are expanding. This is because personal information terminals such as personal computers and network access-compatible cell phones are acceleratingly spreading due to development of information equipments and infrastructures for services network with internet as a core. Further, the market of flat panel displays is expanding to household-use televisions that have conventionally been in the exclusive province of CRTs
Among them, there is an organic electroluminescent device (also referred to hereinafter as “organic EL device”) as a device attracting attention particularly in recent years. The organic EL device is an element that emits light according to electrical signals and is composed of an organic compound as a light-emitting material. The organic EL device inherently has excellent display properties such as wide viewing angle, high contrast and high-speed response. The organic EL device is capable of realizing thin, lightweight and high-definition display devices ranging in size from small to large, and thus attracts attention as an element substituted for CRT or LCD.
Various proposes have been made of full-color display devices using the organic EL device.
For example, as a method of attaining the three basic colors of red (R), green (G) and blue (B) for full-color display, there is a separate forming method in three colors or a method of combining color filters with a white organic EL device.
In the separate forming method in three colors, there is a possibility that higher efficiency can be achieved by arranging three-color suitable materials as coloring materials and by reducing a loss in a circularly polarizing plate. However, the separate forming technique is difficult so that realization of high-definition or large-sized displays is regarded as difficult.
In the method of combining color filters with a white organic EL device to attain the three colors, there is a problem that the luminous efficiency of a white color-emitting material itself is low, and the luminance is reduced by the color filters to about ⅓.
Various improvements have been made in a method of obtaining desired colors by converting the color of light from an organic EL device with a color conversion film, but there are problems such as low efficiency of conversion to red color, and the like.
There is an investigation in which a semitransparent cathode is used as an upper electrode, and by the multiple interference thereof with a reflection film, only light of specific wavelength is picked out of an organic EL device, to realize high color reproducibility. For example, an organic EL device is known wherein a first electrode consisting of a light reflection material, an organic layer provided with an organic light-emitting layer, a semitransparent reflection layer, and a second electrode consisting of a transparent material are laminated in this order and the organic layer is constituted to serve as a resonating region, wherein when the peak wavelength of a spectrum of light to be picked out is λ, the organic EL device is constituted to satisfy the following equation:(2L)/λ+Φ/(2π)=m wherein L is an optical distance, λ is the wavelength of light to be picked out, m is an integer, and Φ is phase shift, and the optical distance L is constituted to be a positive minimum value.
For example, an organic EL display device provided with a micro-cavity (minute resonator) is disclosed (see, for example, Japanese National-Phase Publication No. 2007-503093). Specifically, one pixel is divided into red (R), green (G) and blue (B) sub-pixels each constituting a resonator for resonating emitted light between an optically semitransparent reflection electrode and a reflection film (electrode) and having an organic EL light-emitting layer in common among the sub-pixels, thus making separate forming in three colors unnecessary and a color filter also unnecessary, thereby being assumed to provide a simple full-color display device. A resonant is not arranged in a white sub-pixel region. This is because from the principle of a resonant that resonates light of specific wavelength only, arrangement of a resonant in the white sub-pixel region is not suitable for emitting white light having an emission spectrum in the whole visible range. In the white sub-pixel region, therefore, a transparent electrode is used in place of the optically semitransparent reflection electrode, and separate forming of the white sub-pixel from the R, G and B sub-pixels is thus made necessary. Accordingly, higher resolution is hardly attained, and the process is unfavorably complicated.
A light-emitting device wherein a resonant structure is formed by changing the thickness of an anode for an organic electroluminescence layer, thereby picking out R, G and B lights, is disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2006-269329). As the anode, a transparent electroconductive material such as ITO is used, and a light reflection film is arranged via a transparent insulation film in the lower part of the anode.
In a full-color display device, a white sub-pixel is important for simultaneously achieving rich color reproduction, tone reproduction and low power consumption, and it is desired to solve the problems in arrangement of the white sub-pixel.